An extraordinary sense of foreboding has developed among Syria-watchers over the last few days - a feeling that momentous events are just around the corner. Some even suggest the regime of the president, Bashar al-Assad, could fall within a matter of months.

The reason for this is the unfolding drama in Lebanon that surrounds the UN investigation into the murder of the former prime minister Rafik Hariri.

It is now almost beyond doubt that by the time the chief investigator, Detlev Mehlis, completes his work next month, he will have direct evidence that the assassination was orchestrated from Damascus. If so, the killing of Hariri will probably count as one of the most disastrous own goals in the history of international politics.

Though Mr Mehlis has been discreetly silent about the whole affair, it is not difficult to see where his investigation is heading.

The political context of Hariri's murder, and the most likely motive for it, was set out very clearly in the preliminary UN report last March: Hariri had rebelled against Syrian hegemony in Lebanon and received various threats as a result - one of them allegedly from President Bashar himself.

The Syrian government dismissed the report as being biased, but four Lebanese generals who were in charge of security at the time of the assassination have since been arrested on suspicion of involvement. All, in effect, were working for Syria at the time, and it is clear that if they did organise the killing of Hariri, their orders would have come from Damascus.

A decision last week to open up their bank accounts, along with those of five Lebanese politicians, is expected to cast more light on their dealings with Syria.

According to the Lebanese daily as-Safir, the investigators also have a witness statement giving details of a flat in the village of Bshamoun where Syrian officials allegedly met the four generals to plan the assassination of "a prominent Lebanese figure". Mr Mehlis's team searched the flat last month.

Amid mounting evidence of a Syrian connection, the question is: where in Syria could the order to assassinate Hariri have come from?

If normal procedures were followed, Rustom Ghazaleh, the former head of Syrian intelligence in Lebanon, would probably have delivered it to the Lebanese generals. It is very unlikely, however, that Ghazaleh would have given the order off his own bat: in Syria decisions of that nature have to come from the top - which, in practice, means the inner circle around the president.

Besides President Assad himself, the inner circle at the time is believed to have consisted of:

· Maher Assad, the president's younger brother, who has various military and security functions, including overseeing the presidential guard· General Ghazi Kenaan, the interior minister, who previously spent 19 years as head of military intelligence in Lebanon· General Asef Shawkat, the president's brother-in-law (married to his elder sister, Bushra) who is head of military intelligence. His relations with the president's younger brother have not always been good, and it was reported in 1999 that Maher shot him in the stomach following a quarrel· General Bahjat Suleiman, the hardline head of the internal security division of the General Intelligence Directorate, who reportedly went into semi-retirement last June· Abdel-Halim Khaddam, vice-president and the only Sunni Muslim among the inner circle (the others belong to the minority Alawite sect). Khaddam was on good terms with Hariri and had business dealings with him. He was also the only Syrian official to pay his respects to the family in Beirut after the assassination. In June it was reported that Khaddam was stepping down from the vice-presidency. It is unclear whether he has actually done so, and no successor has been announced.

All national leaders rely to some extent on an inner circle of trusted advisers, but in Syria's case the inner circle is especially important: Bashar is often regarded as a weak president (largely as a result of the way he came to power in 2000 after his father's sudden death) and there are doubts about how much control he really has. Earlier this year, when urged by Crown Prince Abdullah of Saudi Arabia to withdraw Syrian troops from Lebanon as soon as possible, he reportedly replied: "I do not decide everything by myself."

For what it's worth, rumours circulating in Damascus a few months ago and reported by the blogger Joshua Landis of syriacomment.com suggested that among the inner circle, Maher, Shawkat, and Suleiman had favoured assassinating Hariri while Bashar and Kenaan were against.) This is, it should be stressed, rumour rather than established fact.

The 15 Syrians Mehlis hopes to interview shortly in connection with the assassination include at least two from the inner circle - Maher and Kenaan. He also plans to meet Bashar, though Syrian officials maintain that this will be "an audience" rather than an interrogation.

In Bashar's case, the key question will be what exactly he meant during a 10-minute altercation with Hariri last year when he is alleged to have threatened to "break Lebanon over the heads" of Hariri and the Druze leader Walid Jumblatt rather than see his word in Lebanon broken. Was it a threat or just a crude warning?

According to Arab newspaper reports last week, Hariri secretly taped the conversation using a recorder disguised as a pen, which the French president, Jacques Chirac, is said to have given him. Whether this is true or mere disinformation, circulation of the tale was clearly intended to turn up the heat on Damascus ahead of Mehlis's visit.

From Syria's point of view, the least unsatisfactory outcome would be for Mehlis to pin the blame on Ghazaleh, its former intelligence chief in Lebanon, and to sacrifice him as a scapegoat. But already Damascus seems to be working on the assumption that Mehlis will go further and implicate Maher, if not other members of the inner circle.

This would bring the matter to crisis point, since it is very unlikely that Bashar would agree to hand over his own brother, who is also the man responsible for his personal security, for trial.

Last week the president's brother-in-law, Asef Shawkat, was in Paris, apparently trying to negotiate a deal. The general idea seems to be that in exchange for letting Maher off and keeping the regime intact, Damascus would offer greater cooperation on controlling the border with Iraq and perhaps make further sacrifices of Syrian interests in Lebanon.

This is reminiscent of the negotiations with Libya that resolved the standoff over the Lockerbie bombing and other attacks. In the post-9/11 world, however, it may not be so easy to go down that route. The UN security council has already formally declared that Hariri's assassination was an act of terrorism, a move that brings into play all sorts of international agreements created in the wake of September 11. A decision not to pursue suspects in the Hariri case with the utmost vigour would therefore blow a huge hole in President Bush's anti-terrorism strategy.

Although France is playing a key role behind the scenes, the ultimate decision will rest with Washington, where opinion at the moment appears to be divided. Some argue that the Hariri case has created a once-in-a-lifetime opportunity to be rid of the Syrian regime, regardless of other considerations; others say it would not be sensible to create a power vacuum or instability in Syria while there are so many problems in neighbouring Iraq, and that the best course in the meantime is to keep up the diplomatic pressure on Damascus.

The trouble with that is that no one really knows how much pressure the Syrian regime can take, or how it will respond. We could wake up one morning and find that it has simply fallen apart.